Fiber optics is generally concerned with the transmission of light along a transparent fiber structure with a higher refractive index than its surroundings. Using optical fiber equipment in communications systems has several advantages over the use of conventional electronic equipment. For example, optical fiber transmission equipment is compact, lightweight, and potentially inexpensive compared with conventional equipment. Also, transmission over optical fibers does not generate interference and is unaffected by external interference, as compared with conventional equipment.
A typical fiber used in fiber optics consists of core, cladding, and outer protective layer. The core and the cladding have different indices of refraction and together form a guiding region for an optical signal. The difference in refractive indices enables properly directed light to be internally reflected at the core-cladding boundary. Thus, the optical signals will be guided down the core and will not be lost into the cladding. The outer protective layer is used to increase mechanical strength of the fiber and allow the fiber to be more easily manipulated.
All-fiber active devices, such as modulators or switches, could not be constructed using silica-based materials because silica-based devices do not have a second-order non-linearity term in their electric displacement equation. This term is responsible for the change in index of refraction in the fiber due to an applied electric field (the electro-optic effect). However, it has been recently discovered that poling the fiber thermally, with ultra-violet (UV) radiation, or using other methods generates the second-order non-linearity term. Thus, it is now possible to construct all-fiber active devices, such as modulators, from silica-based materials.
Poling can be accomplished a number of ways. In thermal poling, the fiber is heated to a desired temperature, typically 250 to 300 degrees Celsius, and an electric field is simultaneously applied. The fiber is allowed to cool with the electric field still applied. On the other hand, in UV poling, the silica is subjected to a high electric field while being irradiated with a strong source of UV radiation. Both types of poling usually require the silica to be doped with impurities, such as Germanium for the core and Fluorine for the cladding, to increase the electro-optic effect in the fiber to levels needed for the device to function properly. It is also possibly to attain the same result by heavily doping one region while lightly doping (or not doping) the other region.
One example of an active device is a phase modulator. Phase modulators were previously constructed as integrated optical waveguides using lithium niobate. Lithium niobate is a material used extensively in the art for the construction of integrated optical chips. All-fiber modulators can be used anywhere lithium niobate integrated optical waveguide modulators are used. All-fiber devices could replace lithium niobate integrated chips used in high bandwidth optical modulators in CATV and telecommunication systems and sensors, such as, but not limited to closed loop gyroscopes. Other applications could include remote high voltage sensing in the power industry.
Typically, an all-fiber phase modulator is constructed with a fiber having electrodes which apply a voltage across the fiber to change its refractive properties. By applying a voltage, the phase of a signal propagating through the fiber is changed and phase modulation occurs. In order to produce a sufficient electric field in the guiding region both during poling and also for subsequent application of a voltage to the fiber, it is preferred that the electrodes be as close as possible to the fiber core.
One method in the prior art of bringing the electrodes closer to the core consists of polishing an area of the fiber. One poled fiber modulator 10 of this type is shown in FIG. 1. Here, one surface 11 of the fiber is polished, and an electrode 14 is placed on the polished surface near the guiding region. A piece of fused silica 12 is placed on the non-polished fiber surface 13. Another electrode 15 is then placed on the silica 12 to form a modulator 10. FIG. 2 shows another fiber of the prior art which uses polishing. Here, a fiber modulator 20 consists of a fiber 21 with two sides 22 and 23 of the fiber 21 polished and the electrodes 24 and 25 attached on the opposing polished sides 22 and 23.
Another method of bringing electrodes close to the core consists of drilling holes into the fiber and inserting electrodes into these holes. A fiber 30 of this type is shown in FIG. 3. Here, two holes 31 and 32 are formed in the fiber allowing the electrodes to be placed close to the core 33 in the holes 31 and 32.
The prior art, though adequate in some respects, has several shortcomings. For example, the fibers of FIG. 1 and FIG. 2, require polishing. Polishing a fiber is a difficult and costly process because the polishing accuracy must be high due to the small size of the fiber. This is especially difficult and costly for long fiber distances. Therefore, a need exists for a fiber for use in a fiber modulator that does not require costly polishing yet still brings the electrodes sufficiently close to the core for the modulator to function properly.
Additionally, fibers of the type shown in FIG. 3 require electrodes to be inserted into the fiber. Coupling the fiber to other fibers is difficult because the electrodes must exit the fiber, preventing direct butt coupling or fusion splicing to interfacing fibers. Therefore, a need exists for a fiber where the electrodes can be pulled away and do not interfere with the splicing or coupling region.
Also, in fibers of the type illustrated in FIG. 3, it is very difficult to push the fragile electrode wires into the end of the fiber. The cost and time of manufacturing is a significant drawback because of the difficulty of electrode insertion. Thus, a need exists for a fiber which facilitates the positioning of the electrodes.
Furthermore, accurate polishing of a fiber or the insertion fragile wires cannot easily be accomplished over long distances. The cost and time of manufacturing is a significant drawback in the construction of long distance fibers because of the difficulty of electrode insertion over long distances. Therefore, a need exists for a fiber where the electrodes can be reliably and economically placed over long fiber distances.